Mg x Zn 1−x O thin films were grown as metastable alloys via a sputtering technique in order to achieve single-phase wurtzite alloys with deep-UV optical bandgaps. As-grown alloys with Mg composition range 0-72% resulted in optical bandgaps spanning the UV-range of 3.3-4.4 eV. The thermal stability of the alloys was studied via post-growth controlled annealing experiments up to 900 °C. Alloys with low Mg up to 34% were found to be highly stable and retained their optical and material properties; however, alloys with higher Mg, up to 72%, were found to be unstable and were phase separated into wurtzite and cubic structural phases with respective optical bandgaps at ~ 3.5 and 6.0 eV. Both the as-grown and annealed alloys were studied using X-ray diffraction for structural identification, transmission spectroscopy for bandgap analysis, and Raman scattering for mapping the phonon mode-behavior. The experimental value for the solubility limit was found to be ~ 30%. A straightforward model calculation based on the Raman-mode saturation behavior yielded a similar value for the solubility limit of the alloys. The results are discussed in terms of available phase-diagrams for stable-state ceramics alloys that were grown under thermodynamics equilibrium conditions.

To realize the many potential applications of ZnO films, it is vital to produce films with high optical quality that exhibit strong UV luminescence. By combining annealing at an optimal temperature followed by the deposition of a coating, one can achieve a significant enhancement of photoluminescence (PL). The effectiveness of the coating over time is a crucial point to be considered. Three types of coating materials were investigated: MgO, SiO2, and Al2O3. Due to its strong bond energy, MgO was found to be the most effective coating material for passivation of the surfaces of the ZnO films; SiO2 was the second best. The UV-PL intensity of MgO coated ZnO was found to increase by a factor of 52 relative to an uncoated film. The effectiveness of the coatings exhibited a linear correlation with their bond energies and is discussed in terms of competing mechanisms to surface passivation such as the adsorption of OH-groups; these can act as surface traps and diminish the UV-PL intensity. Annealing at 900 °C prior to the deposition of the coating was found to be an important step in realizing the optimal performance of the coating due to the reduction of Zn interstitials accompanied by improved crystallinity. Exposure to the environment, up to 294 days, results in the degradation of the UV-PL of the MgO coated film; this effect was not observed for the film coated with SiO2. This effect is discussed in terms of the strong reactivity of MgO with environmental contaminants from the OH-groups.

Optical and phonon interactions of Ga2O3 thin films with nanocrystalline morphology were studied at extreme temperatures. The films were grown using a sputtering technique and analyzed via temperature response transmission, Raman scattering, and high-resolution deep-UV photoluminescence (PL). Raman modes indicated that the structure corresponds to the β-phase. The optical-gap at the range of 77–620 K exhibited a redshift of ∼200 meV, with a temperature coefficient of ∼0.4 meV/K. The optical-gap at room-temperature is 4.85 eV. The electron–phonon interaction model at that temperature range pointed to a low energy phonon, ∼31 meV, that is involved in the thermal properties of the optical-gap. Detailed Urbach energy analysis indicated that defects are the dominant mechanism controlling the band-edge characteristics even at an elevated temperature regime where phonon dominance is usually expected. Defects are attributed to the disordered forms of graphite that were detected via Raman scattering and to the granular morphology of the film. A deep-UV laser with an above-bandgap exaction line of 5.1 eV was employed to map the PL of the films. The highly resolved spectra, even at room-temperature, show a strong emission of ∼3.56 eV attributed to self-trapped holes (STHs). The STH is discussed and modeled in terms of the self-trapped exciton. Moreover, a very distinct but low-intensity emission was found at 4.85 eV that agrees with the value of the optical-gap and is attributed to bandgap recombination. The intensity ratio between the STH and that of the bandgap was found to be 6:1.